A rack and pinion steering gear unit is used as a mechanism that converts rotating motion that is inputted from a steering wheel to linear motion for applying a steering angle. A steering apparatus that comprises a rack and pinion steering gear unit is well known, and is disclosed in JP2009-184591(A), JP2004-17872(A) and JP61-124471(U).
FIG. 7 illustrates an example of a steering apparatus in which a rack and pinion steering gear unit is assembled. This steering apparatus is a so-called column assist type rack and pinion power steering apparatus. In other words, in order to reduce the operating force for operating the steering wheel 1, auxiliary operating force from a motor 3 that is attached to the middle section of the steering column 2 is applied to the steering shaft. Movement of the steering shaft that rotates as the steering wheel 1 is operated is transmitted to an intermediate shaft 4, and by way of a pinion shaft 5, causes the rack shaft of the rack and pinion steering unit 6 to reciprocate, which steers the steered wheels by way of a pair of left and right tie rods.
On the other hand, FIG. 8 illustrates a so-called pinion assist type rack and pinion power steering apparatus. That is, the operating force for operating the steering wheel 1 is reduced by applying auxiliary operating force from a motor attached to a housing of the steering gear unit 6 to the pinion shaft 5.
Even in column assist or pinion assist type rack and pinion power steering apparatus, the steering gear unit 6, as illustrated in FIG. 9 and FIG. 10, is constructed such that the pinion gear 8 that is formed on part (tip end half) in the axial direction of the pinion shaft 5 engages with the rack teeth 10 that are formed on the front surface of the rack shaft 9 (surface on the top side of FIG. 9 and FIG. 10). A helical rack having a twist angle is used as the rack gear 10, and a helical gear having a twist angle that corresponds to that of the helical rack is used as the pinion gear 8. Part of both the pinion shaft 5 and the rack shaft 9 are housed inside a housing 11.
The housing 11 comprises a main housing section 12 and a sub housing section 13 that are both cylindrical shaped. The main housing section 12 is such that both ends are open, and the sub housing section 13 is provided on the side of part of the main housing section 12, and is such that one end is open. The center axis of the main housing section 12 and the center axis of the sub housing section 13 are in a twisted relationship with each other. The rack shaft 9 is inserted through the main housing section 12 such the displacement in the axial direction is possible, and both end sections thereof protrude from the main housing section 12.
A pair of rack bushings (sliding bearings) 14 that are supported by the portions near both ends of the inner circumferential surface of the main housing section 12 come in sliding contact with the outer circumferential surface of the rack shaft 9, and the rack shaft 9 is able to displace in the axial direction with respect to the main housing section 12 with no backlash movement. The base end sections of a pair of tie rods 7 are connected to both end sections of the rack shaft 9 by way of spherical joints 15. The tip end sections of each tie rod 7 are connected to the tip end sections of knuckle arms (not illustrated in the figure) by a pivot shaft. Due to engagement between the pinion teeth 8 and the rack teeth 10, the rack 9 does not rotate around the center axis of the rack shaft 9.
Due to a reaction force that occurs in the engaging section between the pinion teeth 8 and rack teeth 10 during operation of the steering gear unit 6, a force is applied in a direction that would separate the rack shaft 9 from the pinion shaft 5. Therefore, a mechanism (rack guide) is provided that supports the back surface side of the rack shaft 9, which is the side opposite from the engaging section between the pinion teeth 8 and rack teeth 10, and this mechanism prevents the rack shaft 9 from displacing in a direction away from the pinion shaft 5.
A cylinder section 20 is provided in the housing 11 in the portion in the radial direction of the sub housing section 12 that is on the opposite side from the sub housing section 13, and a rack guide is provided inside the cylinder section 20. For the rack guide, there is a sliding-type rack guide and a rolling-type rack guide. Of these, a sliding-type rack guide 28 that is disclosed in JP2009-184591(A) (FIG. 9 and FIG. 10) comprises: a pressing block 21 that fits inside the cylinder section 20, which on the side that presses the rack shaft 9, has a partial cylindrical concave surface that corresponds to the shape of the back surface of the rack shaft 9; a cover 22 that is screwed onto the opening section of the cylinder section 20; and an elastic section such as a spring 23 or the like that is provided between the pressing block 21 and the cover 22, and is such that the pressing block 21 presses toward the rack shaft 9.
With this construction, together with eliminating backlash in the engaging section between the pinion teeth 8 and the rack teeth 10, it is possible to properly maintain an engaged state between the pinion teeth 8 and the rack teeth 10 regardless of a force that is applied to the rack shaft 9 in a direction away from the pinion shaft 5 as power is transmitted at the engaging section between the pinion teeth 8 and rack teeth 10. In this sliding-type rack guide 28, the partial cylindrical concave surface of the pressing block 21 presses the back surface of the rack shaft 9, so contact between the pressing block 21 and the rack shaft 9 becomes surface contact, and it is possible to effectively prevent backlash movement in the width direction of the rack shaft 9, however, the friction resistance at the area of contact become comparatively large.
On the other hand, a rolling-type rack guide is disclosed in JP2004-17872(A) and JP61-124471(U). In the rolling-type rack guide, by pressing the back surface of the rack shaft with the outer circumferential surface of a pressing roller, backlash in the engaging section between the pinion teeth and the rack teeth is eliminated. In the case of this rolling-type rack guide, the contact state between the pressing roller and the rack shaft is line contact, and it is possible to make the friction resistance at the area of contact comparatively small. Even in the case of a rolling-type rack guide, due to the engagement between the pinion teeth and the rack teeth, a radial load and a thrust load are applied to the pressing roller that presses the back surface of the rack shaft. Therefore, in order to allow the rack shaft to displace smoothly in the axial direction, it is necessary to support the radial load and thrust load that are applied to the pressing roller.
The rack and pinion of steering gear unit that is disclosed in JP2004-17872(A), together with comprising a pair of inclined surfaces on the back surface of the rack shaft that incline in different directions, comprises a pressing roller that is divided into two and that has tapered surfaces that come in contact with each of these inclined surfaces, a support shaft (pin) that supports both ends of the pressing roller such that the pressing roller can rotate freely, and a plate spring that presses the pressing roller in the axial direction; and by pressing the tapered surfaces of the pressing roller against the inclined surfaces of the rack shaft, backlash in the engaging section between the pinion teeth and the rack teeth is eliminated. In this construction, the pressing roller that is divided into two is pressed in the axial direction, so a thrust needle roller bearing is provided between the side surface in the axial direction of the pressing roller and the inside surface of the cylinder section.
In this construction, the inclined surfaces of the rack shaft are pressed in the axial direction of the rack shaft, so the rack shaft may incline with respect to the axis line. Moreover, there is a possibility that manufacturing error will occur between the tapered surfaces of the pressing roller that is divided into two, or that a space will occur between the inner circumferential surface of the pressing roller and the outer circumferential surface of the support shaft. Therefore, in the case of this construction, due to these phenomena, it is not possible to properly maintain a state of engagement in the engaging section between the pinion teeth and the rack teeth, and as a result, there is a possibility that fluctuation will occur in the operation force of the steering wheel, the operator will have an uncomfortable feeling when operating the steering wheel. When the amount that the engagement state in the engaging section between the pinion teeth and the rack teeth shifts from the proper state is excessive, there is also a possibility that partial damage may occur in this engaging section.
FIG. 11 illustrates a rolling-type rack guide 28a that is disclosed in JP61-124471(U). This rolling-type rack guide 28a comprises: a pressing roller 34 having a partial cylindrical concave surface in the circumferential direction that matches the shape of the back surface of the rack 9, a support shaft (pin) 33 that supports the pressing roller 34 so as to be able to rotate freely, a holder 30 for supporting the support shaft 33 on the inside of the cylinder section 20a, and a plate spring 31 that presses the holder 30 in the direction of the rack shaft 10; and by pressing the pressing roller 34 against the back surface of the rack shaft 9, eliminates backlash in the engaging section between the pinion teeth 8 and the rack teeth 10. In this construction, radial loads are supported by a radial needle roller bearing 19 that is provided between the inner circumferential surface of the pressing roller 34 and the outer circumferential surface of the support shaft 33, and thrust loads are supported by a thrust metal bearing 18 that is provided between the inside surface of the holder (support) 30 that supports and secures both end sections in the axial direction of the support shaft 33 and both outside surfaces in the axial direction of the pressing roller 34.